The invention disclosed herein relates generally to illuminated display devices and more particularly to solid state sunlight readable, night vision compatible display devices in which ambient light energy is employed in conjunction with light energy from an artificial light source to illuminate display messages.
There are an increasing number of applications in which display of messages under high ambient light conditions, such as in direct sunlight, is required. Many of these applications also include night vision requirements in which the displayed message must not only be visible under low ambient light conditions, but must not interfere with the ability of personnel to view scenes in low ambient light. Such applications particularly include aircraft cockpit instrumentation and equipment on board marine vessels and space craft and in control towers and portable outdoor ground support systems.
A conventional approach to implementing such displays is to provide back lighting by electrical, electrochromic or other nonambient light sources. Because the back lighting must overcome the brightness of the ambient light, such light sources must be quite intense, which generally corresponds to significant power consumption. In addition, means must be provided for controlling the light intensity to meet night vision requirements.
Only a limited number of kinds of light sources are known which are capable of both producing the required intensity and meeting other practical requirements of the above noted applications. The most common kind of such light sources is a tungsten filament incandescent lamp. Such lamps, particularly when operated at high intensity, have relatively high power consumption and require dissipation of significant amounts of heat. Where a display is incorporated into a device such as a push-button switch, the display/switch can become unpleasantly hot for an operator to touch. In addition, in applications where large numbers of such displays are present in a small area, such as an aircraft cockpit, the use of large amounts of power for this purpose and the attendant requirement for dissipating large amounts of heat is undesirable.
Finally, high reliability of light sources in such applications may be very important. Although relatively long life incandescent light sources are available, incandescent sources are inherently subject to sudden failure, which may be a critical disadvantage. The elevated temperature required for high intensity operation also shortens the life of such light sources, as well as other nearby components.
Other problems which must be dealt with in displays which operate in high ambient light conditions include glare, loss of contrast and feedthrough of the display message when the display device is in an "off" status. Approaches to these problems include incorporation of various filters to absorb ambient light incident upon the face or lens of the device, and/or the incorporation of a shutter which conceals the message when the device is "off". Conventional shutters for such applications have generally been mechanical in nature, and thus involve moving parts which may be complex and difficult to assemble, and inherently suffer from reliability problems.
The applicant has overcome the previously described disadvantages of conventional sunlight readable, night vision compatible displays by providing a unique solid state design which relies on a fluorescing layer energized by either ambient light or light from an electroluminescent panel to illuminate the display.